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How much water per day — a body-weight formula instead of eight glasses
“Eight glasses a day” is one of the most durable pieces of nutrition advice that never had a solid basis. Fluid requirement depends on body mass, activity and temperature — and it can be calculated.
In this guide
Where eight glasses came from
The rule most likely traces back to American recommendations from the 1940s, which suggested around 2.5 litres of water a day for an adult — with a note that most of that quantity is present in the food you eat. The note got lost along the way, and what survived was a round number of glasses to be drunk on top of everything else.
The problem with the rule is not that it is harmful. It is that it accounts for nothing that matters: a person of 55 kg and a person of 95 kg get the same number, and so does a day at a desk and a day with two hours of training in the heat.
How much you actually need
A practical conversion that works well as a starting point:
European recommendations (EFSA) give similar figures expressed differently: 2.0 litres for women and 2.5 litres for men — also as total water intake, and therefore including what comes from food.
Note the phrase “from all sources”. This is where the eight-glasses rule misleads most: food typically supplies 20–30% of daily water. Soup, fruit, vegetables, yoghurt, even cooked pasta — all of it counts.
For someone of 70 kg that usually means about 1.6–1.9 litres to drink, with diet covering the rest. Eat a lot of fruit and vegetables and you will sit near the lower end; live mostly on dry foods and you will sit near the upper.
What counts and what does not
| Drink | Does it count | Note |
|---|---|---|
| Water | Yes, fully | the reference point |
| Tea, herbal infusions | Yes, fully | the diuretic effect is marginal |
| Coffee | Yes | hydrating on net up to 4 cups a day |
| Milk, kefir | Yes | mind the calories |
| Juice, sweetened drinks | Yes, but | sugar can run to hundreds of kcal |
| Alcohol | No | diuretic, dehydrating on net |
Coffee does not dehydrate you
This is the most common misconception on the subject. Caffeine has a mild diuretic effect, but at intakes up to around 400 mg a day — roughly four cups — coffee's water balance stays positive. A cup of coffee is a cup of fluid, and more of it stays than leaves. Regular coffee drinkers also develop tolerance to the effect.
Alcohol is the one exception
Ethanol suppresses vasopressin, the hormone responsible for water retention by the kidneys. The effect is pronounced enough that alcoholic drinks come out net negative: a 500 ml beer leaves less water in the body than it brought in.
How to tell whether you are drinking enough
You do not need a device for this. Three indicators are enough and are surprisingly reliable:
- Urine colour. Pale straw means well hydrated. Dark yellow or amber means too little fluid. Completely clear all day may mean you are drinking more than you need. Note that B vitamins and some medications colour urine regardless of hydration.
- Frequency. Four to seven trips during the day is the typical range.
- Thirst. In a healthy adult this mechanism works and can be trusted. It only loses sensitivity in older people, during hard exercise and in certain medical conditions.
Thirst is sometimes mistaken for hunger — not because the mechanisms are shared, but because both are vague signals from the same region. A glass of water twenty minutes before a meal settles the question and reduces the size of the meal as a side effect.
When you clearly need more
- Training. Around 400–800 ml per hour of exercise, depending on intensity and how much you sweat. Beyond 90 minutes it is worth replacing electrolytes too, not water alone.
- Heat and humidity. Losses through the skin rise several-fold, and in high humidity sweat evaporates poorly, so the body produces more of it.
- Fever, vomiting, diarrhoea. Losses climb sharply and quickly call for electrolyte replacement.
- Altitude above 2,500 m. Faster breathing and dry air increase water lost through the lungs.
- A high-protein diet. Excreting urea costs the kidneys water — at 2 g of protein per kilogram, aim for the upper end of the range.
- Pregnancy and breastfeeding. Requirements rise by roughly 300 ml and 700 ml a day.
Can you drink too much
Yes, though it is rare. Hyponatraemia is the dilution of blood sodium below a safe level — it happens mainly to marathon runners and endurance competitors who drink plain water for hours, replacing volume but not electrolytes. The symptoms — headache, nausea, confusion — are easily mistaken for dehydration, which is dangerous, because it leads to drinking even more water.
In ordinary life the kidneys of a healthy person handle a very wide range of intakes. A sensible upper limit at normal activity is about one litre per hour, and there is no reason to approach it.
Practically: spread your drinking across the day rather than catching up in the evening. A litre downed at once mostly passes straight through — and ruins your sleep on the way.
Common questions
How much water should I drink a day?
About 30–35 ml per kilogram of body mass from all sources, so roughly 2.1–2.5 litres at 70 kg. Since food typically supplies 20–30% of that, around 1.6–1.9 litres is left to drink. EFSA recommendations give similar figures: 2.0 litres for women and 2.5 for men.
Do coffee and tea count toward hydration?
Yes. Caffeine's diuretic effect is mild, and at intakes up to around 400 mg a day — roughly four cups — coffee's water balance stays positive. Regular drinkers also develop tolerance to the effect. The only drink that dehydrates on net is alcohol.
How do I know if I am dehydrated?
The simplest indicator is urine colour: pale straw means well hydrated, dark yellow or amber means a fluid deficit. The second is frequency, typically four to seven times a day. Note that B vitamins colour urine regardless of hydration status.
Is eight glasses of water a day true?
It is a simplification without strong support. The rule most likely comes from advice suggesting 2.5 litres of water a day including what is present in food — the second half of that sentence got lost. Requirements depend on body mass, activity and temperature, so a single number for everyone makes little sense.
How much should I drink during training?
Around 400–800 ml per hour of exercise, depending on intensity, temperature and how heavily you sweat. Past 90 minutes it is worth replacing electrolytes as well, because plain water against large sweat losses dilutes sodium.
Work it out once, then let it run
Plately breaks a photo of your meal into calories and macros, keeps an eye on hydration and charts your weight. The Start plan is free and needs no card.
Open the app →This article is informational and is not medical advice. The formulas below are estimates for healthy adults. If you have a medical condition, take medication, are pregnant or are breastfeeding, set your targets with a clinician rather than a calculator.